1. Field of the Invention
The present invention relates to an epitaxial growing of a semiconductor single crystal layer, and more particularly, to a process of forming an epitaxially grown layer on a single crystal silicon substrate by photo-decomposing a gas raw material with an irradiation of ultraviolet laser light or lamp light, and an apparatus for carrying out the process. The present invention is applied to an epitaxial growth of a silicon (Si), germanium (Ge) or silicon-germanium (SiGe) single crystal layer (thin film).
2. Description of the Related Art
In a production of semiconductor devices such as bipolar transistors and MOS field effect transistors (FETs), an epitaxial growth temperature must be lowered when growing a single crystal epitaxial layer of Si, Ge or SiGe, to avoid the generation of lattice defects, and variations in the impurity diffusion profile in a single crystal (e.g., Si) substrate (wafer), to enable the miniaturization of a semiconductor device, such as a very-large-scale integration (VLSI) device.
Recently, instead of a thermal decomposition (hydrogen reduction) process having a growing temperature of 950.degree. C. or more, a photo CVD process using a photoexciting reaction has been proposed for an expitaxial growth at a temperature lower than the above-mentioned CVD processes. In this case, the epitaxial growth of, e.g., Si, is performed at a temperature of from 600.degree. to 900.degree. C. The photo CVD process however, requires a high temperature annealing treatment (at 900.degree. C. or more) in an ultrahigh vacuum or a hydrogen atmosphere, for removing (cleaning) a natural oxide layer formed on the substrate surface prior to the epitaxial growth, and requires an addition of hydrogen gas to prevent the formation of an oxide layer during the epitaxial growth.
When the high temperature annealing treatment for removing the natural oxide layer is applied to a substrate in which impurity doped (diffused) regions have been formed, the regions (i.e., diffusion profiles) are undesirably expanded. Since the use of hydrogen gas is potentially dangerous (e.g., an explosion could occur) in the operation of an epitaxial growth apparatus, it is preferable to avoid the use of hydrogen gas.
To avoid the high temperature annealing treatment, it has been proposed that SiH.sub.2 F.sub.2 gas be added in the hydrogen gas to generate SiF radicals therefrom at a low temperature of 100.degree. to 300.degree. C., as these radicals accelerate the removal of the natural oxide layer. Furthermore, the addition of SiH.sub.2 F.sub.2 gas to the gas raw material generates SiF radicals which prevent the formation of the oxide layer during the epitaxial growth (cf. e.g., A. Yamada et al., Extended Abstracts of the 18th Conference on Solid State Devices and Materials, Tokyo, 1986, pp. 217-220). SiH.sub.2 F.sub.2 gas, however, has a spontaneous combustion property and is potentially more dangerous than hydrogen, and therefore, it is necessary to avoid the use of SiH.sub.2 F.sub.2 gas.